1-(2′-Pyridylazo)-2-naphtholate (PAN) complexes of rhodium(III): Synthesis, structure and spectral studies

Article abstract of DOI:10.1016/j.poly.2009.12.001

The ligating properties 1-(2′-pyridylazo)-2-naphthol (HPAN) toward Rh(III) have been examined. The reaction of RhCl₃·3H₂O with HPAN in presence of excess PPh₃ afforded trans-[Rh(PAN)Cl(PPh₃)₂]PF₆

Full text of DOI:10.1016/j.poly.2009.12.001

Polyhedron 29 (2010) 1015–1022
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Polyhedron
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1-(20-Pyridylazo)-2-naphtholate (PAN) complexes of rhodium(III): Synthesis,
structure and spectral studies
*
Kausikisankar Pramanik , Basab Adhikari
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
The ligating properties 1-(20-pyridylazo)-2-naphthol (HPAN) toward Rh(III) have been examined. The
reaction of RhCl3ꢀ3H2O with HPAN in presence of excess PPh3 afforded trans-[Rh(PAN)Cl(PPh3)2]PF6
(3PF6). Intermediate cis-[Rh(PAN)Cl2(PPh3)] (4) has also been isolated. Solid state structures were authen-
ticated by X-ray analyses revealing that monoanionic PAN is coordinated to rhodium in meridional fash-
ion. Both the compounds were spectroscopically characterized in both solution and solid states, which
include IR, NMR (1H and 31P), and optical spectra. The diamagnetic complexes show multiple CT transi-
tions in the visible region. Low-energy transitions (k ꢁ 550–650 nm) occurred in the absorption spectra
are predominantly ligand centered in nature. The rhodium(III)–PAN compounds are red emissive
(kem ꢁ 650 nm) at room temperature and the nature of the emission level is probably an ILCT level. Com-
plexes are electro-active in acetonitrile and display irreversible oxidative and reductive waves and these
responses are ascribed to be PAN ligand centered in character.
Article history:
Received 4 August 2009
Accepted 2 December 2009
Available online 5 December 2009
Keywords:
Rh(III) complexes
1-(20-Pyridylazo)-2-naphthol
Crystal structures
Spectral studies
Ligand redox
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
by Tschitschibabin [12,13]. Subsequently, Cheng and Bray reported
PAN as a chelating agent (metallochromic indicator) for the first
The chemistry of rhodium containing multidentate ligands
incorporating -acceptor azo function is of abiding interest. Nota-
time in 1955 [14]. Although, ample precedent exists for PAN as
an analytical reagent for both transition and main group metals
and used in qualitative as well as semiquantitative analyses [15–
22], the structurally authenticated complexes are scarce and lim-
ited only to Cu(II) [23,24], Co(II and III) [25], Ru(II) [26] and Re(III)
[27] centers. The first structural characterization of a copper com-
plex was done in 1967, revealing the ligand binds to metal center
via dissociation of the naphtholic proton, as a tridentate monoan-
ionic N,N,O-donor forming two contiguous five-membered che-
lates (2) [23]. Significantly, the PAN ligand comprises two kinds
p
ble examples embrace the complexes of arylazothioethers [1],
arylazothiolates [1], arylazophenolates [2,3], arylazoamines [4]
and arylazoimines [5]. A variety of remarkable features showed
by the complexes of this metal with tridentate azo ligands which
include interesting coordination modes and molecular structures,
potential sites of facile electron transfer, metal assisted C-hetero
atom bond scission, interesting electronic structure due to the
*
p orbitals, photophysical behavior of delo-
presence of low-lying
calized transition metal chelates, and so forth. As the close envi-
ronment about the metal ion governs the chemistry of the
metal–ligand framework and a small change in ligand architecture
can strongly influence the properties of the complexes, coordina-
tion of rhodium by azo ligands of different types has been of signif-
icant importance. It is worth noting that the structurally
authenticated rhodium compounds with analogous tridentate li-
gands containing pyridylazo moiety are scarce [6,7].
of donors viz. the potent
stabilizing low oxidation states [28–30] and the strong
p
-acceptor pyridylazo moiety and hence
p-donor
naphtholato-O atom, a familiar stabilizer of higher oxidation states
of transition metal [31,32]. Notably, rhodium offers two stable oxi-
dation states viz. (I) and (III), while the intermediate (II) state is
unstable and highest possible (IV) state is sparse [33]. It is now
of interest to recognize the valence preference of the title ligand to-
ward rhodium.
With this background in mind, we are concerned with synthe-
sizing new pyridylazophenolate molecules since there are only a
few reports of rhodium compounds where PAN was utilized as li-
gand [8–11] and none of these are structurally characterized. The
1-(20-pyridylazo)-2-naphthol ligand (HPAN, 1) was synthesized
In the present paper, we report the synthesis, spectroscopic and
electrochemical properties of two rhodium(III) complexes coordi-
nated with PAN ligand, trans-[Rh(PAN)Cl(PPh3)2]PF6 (3PF6) and
cis-[Rh(PAN)Cl2(PPh3)] (4) obtained in the reaction of RhCl3ꢀ3H2O
with 1-(20-pyridylazo)-2-naphthol (HPAN) in presence of PPh3.
The voltammetric data and electronic spectral assignments have
also been rationalized by Density Functional Theory (DFT) calcula-
tions. The structures of both complexes have been determined by
single crystal X-ray crystallography.
* Corresponding author.
E-mail address: kpramanik@hotmail.com (K. Pramanik).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.12.001

1016
K. Pramanik, B. Adhikari / Polyhedron 29 (2010) 1015–1022
was refluxed for 4 h. After concentrating the volume to about 15 ml
under reduced pressure, saturated ethanolic solution of NH4PF6
was added and the mixture stirred for few minutes until precipita-
tion of the green 3PF6. Precipitate was filtered, washed thoroughly
with water and chromatographic purification was made by the
above procedure. Yield: 93%.
2.5. Physical measurements
1H NMR spectral measurements were carried out on a Bruker FT
300 MHz spectrometer with TMS as an internal standard. 31P NMR
spectra were measured on a Bruker AMX 400 MHz spectrometer
operating at 161.92 MHz. UV–Vis spectra were recorded on a Perk-
inElmer LAMBDA 25 spectrophotometer. IR spectra were recorded
on a PerkinElmer L-0100 spectrometer. The elemental analyses (C,
H, N) were performed with a PerkinElmer model 2400 series II ele-
mental analyzer. Room temperature emission spectra were re-
2. Experimental
2.1. Materials
1-(20-Pyridylazo)-2-naphthol, triphenylphosphine and cresyl
violet perchlorate were purchased from Aldrich Chemical Co.
RhCl3ꢀ3H2O was purchased from Arora Matthey Limited and trieth-
ylamine from Merck, India. All solvents were used as received
without purification. [RhCl(PPh3)3] was synthesized by following
a reported procedure [34].
corded on
a PerkinElmer LS 55 Luminescence spectrometer.
Electrochemical measurements were carried out at 25 °C with Vers-
aStat II Princeton Applied Research potentiostat/galvanostat under
argon atmosphere. The cell contained a Pt working electrode and a
Pt wire auxiliary electrode. Tetraethylammonium perchlorate
(NEt4ClO4) was used as supporting electrolyte and the potentials
are referenced to the Ag/AgCl electrode without junction correction.
2.2. Preparation of [Rh(PAN)Cl(PPh3)2]PF6 (3PF6)
To a 35 mL ethanolic solution of HPAN (95 mg, 0.382 mmol),
triethylamine (58 mg, 0.574 mmol) was added. Then ethanolic
solution (5 mL) of RhCl3ꢀ3H2O (100 mg, 0.380 mmol) and triphen-
ylphosphine (400 mg, 1.527 mmol) were added successively and
the mixture was refluxed for 4 h. After concentrating the volume
to about 15 mL under reduced pressure, saturated ethanolic solu-
tion of NH4PF6 was added and stirred for 15 min to precipitate
the green complex. The precipitate was filtered and washed thor-
oughly with water. Upon chromatographic purification of the solid
on neutral silica gel (60–120 mesh) column with 5:1 toluene–ace-
tonitrile mixture as eluent, green 3PF6 was obtained. Yield 81%.
Anal.Calc. for C51H40N3OP3F6ClRh: C, 57.98; H, 3.79; N, 3.98. Found:
2.6. Crystallographic studies
The single crystals suitable for X-ray crystallographic analysis of
both the complexes 3PF6 and 4 were obtained by slow diffusion of
dichloromethane solution of the complexes into hexane. X-ray
intensity data for 3PF6 and 4 were collected at 293 K o0n Bruker
SMART APEX CCD diffractometer (Mo Ka, k = 0.71073 ÅA). Metal
atoms were located by direct methods, and the rest of the non-
hydrogen atoms emerged from successive Fourier synthesis. The
structures were refined by full-matrix least squares procedure on
F2. All non-hydrogen atoms were refined anisotropically. All the
hydrogen atoms were included in calculated positions and were
treated as riding atoms using SHELXL default parameters. Metal
atoms were located and solved by direct methods using the pro-
gram SHELXS-97 [35]. The refinement and all further calculations
were carried out using SHELXL-97 [36]. Calculations were performed
using the SHELXTL v.6.14 program package [37]. Molecular structure
plots were drawn using ORTEP [38]. Thermal ellipsoids were drawn
at the 35% probability level.
C, 57.64; H, 3.83; N, 3.87%. IR (KBr, cmꢂ1): 1326
m(N@N), 694 and
519 (Rh–P), 840
m
m
(P–F). 1H NMR (300 MHz, CDCl3): d = 8.72 (d,
J = 8.5 Hz), 7.97 (d, J = 7.9 Hz), 7.81 (t, J = 8.6 Hz), 7.67 (t,
J = 8.4 Hz), 7.54 (d, J = 4.6 Hz), 7.49–7.42 (m), 7.33–7.16 (m), 6.48
(t, J = 6.5 Hz), 6.43 (d, J = 9.4 Hz). 31P NMR (161.92 MHz, CDCl3):
1
1
d = 19.43 (d, JRh–P = 87.4 Hz, PPh3), ꢂ144.07 (septet, JP–F
=
725.4 Hz, PF6ꢂ).
2.3. Preparation of [Rh(PAN)Cl2(PPh3)] (4)
2.7. Computational details
To a 40 mL ethanolic solution of HPAN (95 mg, 0.382 mmol), tri-
ethylamine (58 mg, 0.574 mmol) and triphenylphosphine (105 mg,
0.401 mmol) were added. Then ethanolic solution (5 mL) of
RhCl3ꢀ3H2O (100 mg, 0.380 mmol) was added and the mixture
was refluxed for 2 h to afford a green solution. Solvent was evapo-
rated under reduced pressure and the solid mass was subjected to
column chromatography on neutral silica gel (60–120 mesh) col-
umn. With 3:1 toluene–acetonitrile mixture as eluent, green 4
was obtained. Yield: 68%. Anal. Calc. for C33H25N3OPCl2Rh: C,
57.89; H, 3.65; N, 6.14. Found: C, 57.62; H, 3.70; N, 6.05%. IR
All calculations were performed using Density Functional The-
ory (DFT) [39] with Becke’s three-parameter hybrid exchange func-
tional [40] and the Lee–Yang–Parr nonlocal correlation functional
(B3LYP) [41]. The atomic coordinates of all the complexes were ta-
ken from their X-ray structures. All calculations were performed
with the GAUSSIAN 03 (G03) program [42]. The 6-31+G basis set
was used for carbon, nitrogen and oxygen, 6-31+G(d,p) for chlorine
and phosphorus and 6-31G was used for hydrogen. LANL2DZ basis
set with Hay and Wadt effective core potential [43,44] was used for
rhodium. Orbital diagrams were generated at isosurface value of
0.02 using GAUSSVIEW 3.09.
(KBr, cmꢂ1): 1315
m(N@N), 683 and 519 m
(Rh–P). 1H NMR
(300 MHz, CDCl3): d = 8.88 (d, J = 8.34 Hz), 8.25 (d, J = 5.42 Hz),
7.67 (t, J = 7.86 Hz), 7.63–7.45 (m), 7.39 (d, J = 7.72 Hz), 7.32–7.15
(m), 6.77 (t, J = 6.23 Hz), 6.71 (d, J = 9.40 Hz). 31P NMR
1
(161.92 MHz, CDCl3): d = 27.24 (d, JRh–P = 115.0 Hz, PPh3).
3. Results and discussion
2.4. Conversion of 4 to 3PF6
3.1. Synthesis
To a 30 mL ethanolic solution of 4 (50 mg, 0.073 mmol), tri-
phenylphosphine (40 mg, 0.152 mmol) was added and the mixture
In the present work, which has originated from our interest in
the chemistry of rhodium in different coordination environments

K. Pramanik, B. Adhikari / Polyhedron 29 (2010) 1015–1022
1017
[1], we have chosen HPAN (1) as the prime ligand type. RhCl3ꢀ3H2O
has been selected as the starting material with the expectation that
metal halide alone is competent to furnish halo-phosphine ana-
logue of [RhIII(L)Cl(PPh3)2] (L = 2-(phenylazo)thiolate, a tridentate
azo ligand) in presence of PPh3 [1]. Treatment of an alcoholic solu-
tion of RhCl3ꢀ3H2O with 1 and triethylamine in presence of excess
PPh3 under refluxing condition has indeed afforded [Rh(PAN)Cl-
(PPh3)2]PF6 (3PF6) as an emerald solid in good yield (Scheme 1).
Presence of triethylamine ensures the deprotonation of HPAN in
solution. The title ligand is coordinated as tridentate uninegative
mode. Addition of NH4PF6 is thus needed to satisfy the primary va-
lence of trivalent rhodium during the course of the synthesis.
It is worth noting that the formation of the cationic complex
3PF6 was diminished drastically when reaction was performed
with 1 equivalent of PPh3 coligand relative to both 1 and RhCl3ꢀ
3H2O. In that case, green [Rh(PAN)Cl2(PPh3)] compound 4 was ob-
tained as the major product (Scheme 2). Significantly, the relative
proportion of the products (3+ versus 4) appears to be sensitive to-
ward the sequence of addition of metal halide and triphenylphos-
phine in due course of the reaction to the alcoholic solution of the
dye. Prior addition of PPh3 (1 equivalent) to the metal salt always
affords 4 exclusively whereas addition of PPh3 (1 equivalent) to a
previously mixed 1:1 solution of RhCl3 and PAN leads to the forma-
tion of both 4 and 3+ in comparable yields. Although the neutral
compound 4 was not observed with RhCl3 in the presence of excess
PPh3, Wilkinson’s catalyst, viz. [RhCl(PPh3)3], alone is competent to
furnish 4 in analogous conditions in moderate yield; reflecting the
fascination of PAN toward rhodium(III), a borderline metal center
(Scheme 2) [45]. Albeit the underlying redox process is not clear
to us, the plausible oxidant is most likely to be the proton [46].
Notably, the monophosphine complex 4 is converted almost quan-
titatively to bisphosphine analogue 3+ in presence of twofold ex-
cess of PPh3 (Scheme 2), revealing the superior stability of the
bisphosphine product over the monophosphine one. In rhodium–
phosphine chemistry coordination of single tertiary phosphine is
less common, particularly when the corresponding bisphosphine
product is formed along the reaction coordinate. It is now of inter-
est to stabilize and isolate the intermediate rhodium compounds
coordinated with single phosphine molecule as well as to explore
their chemistry during the synthesis of the corresponding bisphos-
phine product. To the best of our knowledge, only a few instances
are reported where monophosphine rhodium precursors are iso-
lated, which ultimately afford more stable bisphosphine analogues
in due course of the reaction [1]. The analytical data for these com-
plexes are in good agreement with the above molecular formula. In
both the reactions, it has been observed that the HPAN behaves as
a bifunctional tridentate ligand.
3.2. Crystal structure
1. 1 eq. RhCl .3H O
3
2
The X-ray crystal structures of 3PF6 and 4 were determined to
establish the geometries about the metal ions as well as the bond-
ing modes of the PAN and the coligands. Single crystals of the
respective complex suitable for X-ray analysis were grown by slow
diffusion of hexane into dichloromethane solution at room temper-
2. 4 eq. PPh
3
3. NH PF
4
6
[Rh(PAN)Cl(PPh ) ]PF
3 2
HPAN + NEt
1
6
3
(EtOH)
3PF
6
ꢀ
ature. Both compounds crystallize in the triclinic space group P1.
P
The molecular views and atom numbering schemes of these
complexes appear in Figs. 1 and 2, respectively. The selected bond
distances and angles are given in Table 1. Experimental crystallo-
graphic parameters are summarized in Table 2.
The structure of 3PF6 reveals the presence of mononuclear
trans-[Rh(PAN)Cl(PPh3)2]+ cation, hexafluorophosphate anion and
one dichloromethane molecule as solvate in the asymmetric unit.
In this complex, the central rhodium(III) is surrounded with a
N2OClP2 distorted octahedral coordination environment by triden-
tate monoanionic PAN ligand (N, N, O), two trans-PPh3, and one
chloride ligands. The title ligand coordinates to rhodium(III) in a
meridional fashion using the pyridyl-nitrogen N1, azo-nitrogen
N
N
Cl
O
N
N
O
N
P
=
N
O
Rh
P
= PPh
3
N
+
3
Scheme 1. Synthesis and structure of cationic [RhIII(PAN)Cl(PPh3)2]+ complex.
[Rh(PAN)Cl (PPh )]
2
3 4
1. 1 eq. PPh
2. 1 eq. RhCl
3
3
(major)
Wilkinson's Catalyst
(EtOH)
HPAN + NEt
1
HPAN + NEt
1
3
3
+
(EtOH)
[Rh(PAN)Cl(PPh ) ]
PF
3 2 3
6
(minor)
P
P
P
N
N
N
N
Cl
O
Cl
Rh
Rh
(EtOH)
O
P
Cl
+
4
3
Scheme 2. Synthesis and structure of neutral [RhIII(PAN)Cl2(PPh3)] complex and the conversion of 4–3+ compound.

1018
K. Pramanik, B. Adhikari / Polyhedron 29 (2010) 1015–1022
Table 1
Selected bond lengths and angles for 3PF6ꢀCH2Cl2 and 4ꢀCH2Cl2.
Bonds
3PF6
4
Bond lengths (Å)
Rh1–N1
Rh1–N3
Rh1–O1
Rh1–P1
Rh1–P2
Rh1–Cl1
Rh1–Cl2
N2–N3
2.029(2)
1.945(2)
2.057(2)
2.421(1)
2.389(1)
2.385(1)
2.012(4)
1.931(4)
2.042(3)
2.293(1)
2.358(1)
2.374(1)
1.293(5)
1.303(3)
Bond angles (°)
N1–Rh1–O1
N3 – Rh1–Cl1
P1–Rh1–P2
161.12(8)
177.22(6)
174.44(2)
161.37(14)
175.28(12)
P1–Rh1–Cl2
N1–Rh1–N3
O1–Rh1–N3
176.58(5)
79.95(15)
81.45(14)
79.49(9)
81.70(8)
Table 2
Crystal data and structure refinement parameters for 3PF6ꢀCH2Cl2 and 4ꢀCH2Cl2.
Parameters
3PF6ꢀCH2Cl2
4ꢀCH2Cl2
Empirical
formula
[C51H40ClN3OP2Rh]PF6ꢀCH2Cl2 C33H25Cl2N3OPRhꢀCH2Cl2
Formula weight
T (K)
Crystal system
Space group
a (Å)
1141.06
273(2)
769.27
273(2)
triclinic
Fig. 1. Molecular structure of 3PF6. Counter anion (PF6ꢂ), solvate (CH2Cl2) and H
atoms are omitted for clarity.
triclinic
ꢀ
ꢀ
P1
P1
11.118(2)
13.600(3)
17.279(4)
91.01(3)
104.56(3)
94.01(3)
2
1.503
0.657
1156
10.889(1)
12.420(1)
14.137(2)
90.48(1)
104.60(1)
112.93(1)
2
1.510
0.900
776
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
Z
Dcalc (mg mꢂ3
)
l
(mmꢂ1
F(0 0 0)
)
h Range (°)
Measured
reflections
Unique
1.90–25.00
19 638
1.50–25.94
16 818
8798 (0.0165)
6537 (0.0467)
reflections
(Rint
)
a
R [I > 2
r
(I)] R1
,
0.0330, 0.0893
0.0395, 0.0941
1.080
0.0514, 0.1261
0.0880, 0.1454
1.040
b
wR2
R (all data) R1,
wR2
Goodness-of-fit
(GOF) on F2
CCDC No.
703322
703321
Fig. 2. Molecular structure of 4. Solvate and H atoms are omitted for clarity.
P
P
a
R1
=
|Fo| ꢂ |Fc|/ |Fo|.
P
P
wR2 = [ w(Fo2 ꢂ Fc2)2/ w(F2o)2]1/2
.
b
N3, and naphtholato-oxygen O1 so as to form two adjoining five-
membered chelates.
Crystallographic analysis of the neutral complex 4 reveals a dis-
torted octahedral structure with meridional N, N, O coordination of
PAN acts as a tridentate monoanionic donor. The coordination
sphere of the metal atom is completed by one triphenylphosphine
and two cis-chloride ligands. The asymmetric unit also contains
one CH2Cl2 molecule as a solvent of crystallization.
are comparable. Nonetheless, several significant features are ob-
served for the mono cationic 3+ and neutral 4 complexes, notably:
(i) the Rh–N(azo) lengths (average 1.938(3) Å) are noticeably
shorter by ca. 0.08 Å than the Rh–N(py) lengths (average
2.020(3) Å) in both compounds, indicating the stronger Rh(t2) ?
*
N(azo
p
) back donation. (ii) Appreciable elongation of N–N
The bond distances (Table 1) indicate strong Rh(d )–PAN(p )
lengths of azo (–N@N–) chromophores (average lengths
1.298(4) Å) in the coordinating ligand, while that in uncoordinated
azo ligands lies near 1.25 Å [1,47]. Coordination of PAN to rho-
dium(III) can lead to a decrease in the N–N bond order due to both
p
p
interactions in both complexes. The equatorial plane in pseudooc-
tahedral geometries comprises PAN and one chloride ligand. The
mean deviations for the planes including N1, N3, O1 and Cl1 of
3+ and 4 are 0.022 and 0.026 Å, respectively and for that including
the metal atom, N1, N3, O1, Cl1 and Rh1 are 0.018 and 0.030 Å,
respectively. Metrical parameters associated with Rh–N(py), Rh–
N(azo), Rh–O(naphthalato) and N–N linkages in both complexes
r
-donor and p-acceptor character of the ligand, the later character
having a more pronounced effect. (iii) Ru1–P1(trans to chloride li-
gand) length of 4 is significantly shorter (ꢃ0.11 Å) as compared to
the average of Ru–P distances in 3+ (average 2.405(1) Å) (Table 1),

K. Pramanik, B. Adhikari / Polyhedron 29 (2010) 1015–1022
1019
signifying that RhIII(t2) ? PPh3(d/
r ) back-bonding severely im-
*
pedes in presence of
p-acceptor PPh3 ligand in the trans position.
This difference in bond distances is also reflected in the 31P NMR
coupling constants, where JRh–P = 115.0 and 87.4 Hz in 4 and 3+,
1
respectively (vide supra). (iv) Rh1–Cl1(trans to azo) length is short-
er by ca. 0.02 Å than Rh1–Cl2(trans to PPh3 ligand) length of 4,
which is attributed to the superior
p-acidity vis-à-vis trans influ-
ence of PPh3 as compared to azo function [1]. This disparity in
bonding pattern between two chloride atoms of 4 is reflected in
the notable labileness of Rh1–Cl2(trans to PPh3) bond and explain
facile substitution of Cl2 atom with strong
p-acid PPh3 coligand
(vide infra) during conversion of 4 to 3+. (v) The N, O chelate bite
angles of the monoanionic ligand (average 81.6(1)°) are marginally
larger than the N, N chelate bite angles of pyridylazo moiety (aver-
age 79.7(1)°) in both complexes.
3.3. Spectral studies
3.3.1. IR spectra
Infrared spectra of both the complexes exhibit many sharp and
strong vibrations within 1650–400 cmꢂ1. A sharp vibration near
1320 cmꢂ1 of the complexes is assigned to the
mN=N. The observed
lowering of mN=N values by nearly 100 cmꢂ1 in complexes as com-
*
pared to that of 1 is consistent with the Rh(III) ?
p
(azo) back-
bonding [6,30]. Strong bands near 520 and 690 cmꢂ1 are indicative
of Rh–PPh3 ligation in the complexes. An additional intense feature
at 840 cmꢂ1 is observed in the case of 3PF6, which is attributed to
the hexafluorophosphate ion.
3.3.2. NMR spectra
The synthesized complexes provide satisfactory elemental anal-
yses and are diamagnetic, which corresponds to the trivalent rho-
dium (low-spin d6, S = 0) in these complexes. 1H NMR spectra of
these compounds have been recorded in CDCl3 and the spectra
are given in the Supplementary data (Fig. S1). The ranges of proton
signals for the free HPAN and coordinated PAN ligands appeared
almost in the similar region. The aromatic region (d 6.4–8.9) is
rather complex in nature owing to overlap of signals arising from
the PPh3 protons and PAN ligands, of which the most deshielded
doublet near 8.8 ppm is assignable to the proton attached with
the C1 atom (Figs. 1 and 2). 1H NMR spectra of both the complexes
show multiple signals for the PPh3 protons within 7.1–7.7 ppm
range. Spectra of 3PF6 and 4 show 1:2 and 1:1 relation of ligand/
triphenylphosphine signals, respectively; confirming the number
of PPh3 molecule in complexes. Absence of the naphtholic proton
signal (appears as a distinct singlet at d = 15.8 ppm in the free li-
gand) in complexes authenticates the coordination of naphthola-
to-O atom. The 31P NMR spectra of the mono and bisphosphine
complexes exhibit only single doublet resonances (103Rh nucleus,
I = 1/2), indicating equivalent phosphine environment (i.e., trans
dispositions of PPh3 groups) in the latter compound (Fig. 3). The
formation of monophosphine compound 4 results in a significant
deshielding (d ꢃ27 ppm for PPh3 trans to chloride in 4 versus d
Fig. 3. 31P NMR spectra of (a) cationic complex 3+ and the counter anion PF6 is
shown in the inset and (b) neutral complex 4 in CDCl3.
ꢂ
phine intermediate 4 is moderately soluble in nonpolar organic
solvents such as benzene and toluene. Electronic spectra of the
complexes have been recorded in acetonitrile (Fig. 4) at room tem-
ꢃ19 ppm for PPh3 trans to
p
-acid ligand PPh3 in 3+) in the 31P sig-
1
nals. The higher JRh–P value of monophosphine compound by ca.
28 Hz in comparison to that of bisphosphine analogue is indicative
of the shorter Rh–P length in the former compound (vide infra). In
3PF6, a septet pattern is observed at ꢂ144 ppm due to the spin–
spin coupling of 19Fꢂnuclei (I = 1/2) with the 31P nucleus, reflecting
the presence of PF6 as counter anion (Fig. 3).
3.3.3. UV–Vis spectra
Both the complexes are soluble in polar organic solvents such as
ethanol, acetonitrile, dichloromethane, chloroform, acetone and so
forth, furnishing green solutions but insoluble in hydrocarbon sol-
vents like n-hexane, cyclohexane and heptane. The monophos-
Fig. 4. UV–Vis spectra of 3PF6 (- - - -) and 4 (—) in acetonitrile. Emission spectra of
3PF6 (- - - -) and 4 (—) in acetonitrile are shown in the inset.

1020
K. Pramanik, B. Adhikari / Polyhedron 29 (2010) 1015–1022
Table 3
UV–Vis, emission spectral and cyclic voltammetric data.
c
Compound
UV–Vis dataa
kmax (nm) (
, Mꢂ1 cmꢂ1
Emission spectral datab
kmax (nm)
Quantum yield (
U
)
Cyclic Voltammetric datad (V)
e
)
[Rh(PAN)Cl(PPh3)2]PF6
[Rh(PAN)Cl2(PPh3)]
624(11 790), 579(8760),
440(8380), 325(25 825)
640(10 950), 593(9120),
436(11 150), 278(28 596)
648
661
8.3 ꢄ 10ꢂ4
1.6 ꢄ 10ꢂ3
1.08e, ꢂ0.48f
1.60e, ꢂ0.54f
a
b
c
d
e
f
In acetonitrile.
In acetonitrile, excitation at 580 nm.
In deoxygenated acetonitrile.
Solvent, acetonitrile; supporting electrolyte, TEAP; reference electrode Ag/AgCl; scan rate, 50 mV sꢂ1
Anodic peak potential (Epa) value.
.
Cathodic peak potential (Epc) value.
perature. Multiple low-energy transitions are the characteristics of
the spectra (Table 3) and these excitations are believed to be due to
charge-transfer transitions involving the three-coordinated 2-
(pyridylazo)naphtholate and other coligand orbitals. The spectral
patterns of the compounds are similar except the slight hypsochro-
mic shift of the low-energy excitations (550–650 nm) by ca. 15 nm
in case of 3PF6 relative to 4, which is ascribed to the weaker
charge-transfer transitions upon second phosphine coordination.
Notably, the low-energy transitions in rhodium(III) complexes
are less common [6,48] and may have usefulness with regard to
the development of photocatalysts active in the long-wavelength
region of the visible spectrum. To gain insight into the nature of
FMOs, we performed Density Functional Theory (DFT) calculations
with the crystallographic parameters. The isodensity plot from
HOMOꢂ1 to LUMO+1 for 3PF6 and 4 is illustrated in Fig. 5 and
composition of these molecular orbitals is given in Table S1. The
calculations have indicated that the top three occupied orbitals
(HOMO, HOMOꢂ1 and HOMOꢂ2) as well as low-energy acceptor
orbitals (LUMO, LUMO+1, LUMO+2) in both the complexes are pre-
dominantly centered on PAN ligand and coligand (Cl and PPh3)
orbitals with only small contributions from the metal orbitals.
Significantly, Rh-d orbitals appear to contribute very little electron
density to either the HOMO or LUMO. Additionally, presence of
low-energy MLCT and LMCT states are unexpected for these com-
pounds since Rh(III) is rather redox inert [49]. These results tempt
us to propose that these low-energy transitions involve orbitals
that are predominantly ligand in character and originated primar-
ily from the coordinated PAN ligand part.
3.3.4. Emission spectra
Rhodium(III) polyazine complexes have been extensively stud-
ied for their photophysical property in recent years though they
usually do not exhibit photoluminescence at room temperature
[50–56]. To the best of our knowledge, no report about photolumi-
nescence of rhodium(III)–PAN (a typical azo-imine kind of ligand)
compounds is documented in the literature. Here we present the
room temperature photoluminescence spectra for both rho-
dium(III)–PAN compounds (Table 3). The complexes 3PF6 and 4
are luminescent in acetonitrile solution with a broad emission
band centered at k = 648 nm (
U
= 8.3 ꢄ 10ꢂ4) and k = 661 nm
(U
= 1.6 ꢄ 10ꢂ3), respectively, after excitation at k = 580 nm. Emis-
The HOMO of 3PF6 consists principally of a mixture of PAN-
p
p
sion quantum yields were determined at 25 °C in deoxygenated
acetonitrile solution (argon bubbling for 30 min) with cresyl violet
perchlorate in methanol as a standard (Ur = 0.54) [57]. The emis-
sion quantum yield for complexes (Uu) was calculated by the
well-known equation given below:
and phenyl- of PPh3, and that of 4 is localized largely on PAN-
p
and Cl-p orbitals; with major contribution (ꢃ75%) from the PAN li-
gand in both cases. LUMO for both the compounds are distributed
almost exclusively (ꢃ90%) over the three-coordinated PAN ligand.
Fig. 5. Selected FMOs of 3PF6 (top) and 4 (bottom) (isosurface cutoff value = 0.02). Color code for atom: grey, C; blue, N; orange, P; red, O; green, Cl; aquamarine, Rh. (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

K. Pramanik, B. Adhikari / Polyhedron 29 (2010) 1015–1022
1021
2
than in uncoordinated HPAN reflecting the expected stabilization
*
Uu
¼
UrðAr=AuÞðIu=IrÞðnu=nrÞ
ð1Þ
of the
p MO upon coordination [62]. The redox noninnocence of
where Au, Ar are the absorption values of unknown and reference; Iu,
Ir are the integrated emission areas of unknown and reference; and
nu, nr are the refractive indices of the solvents containing unknown
and reference [58]. The experimental red emission spectra are de-
picted in the inset of Fig. 4. Notably, platinum group metal–phos-
phine complexes hardly show emissive property at room
temperature [59], the synthesized complexes do emit. Nonetheless,
the emission intensity decreases in presence of PPh3 group as the
monophosphine derivative 4 appears to be nearly doubly emissive
relative to bisphosphine compound 3PF6. MLCT character of the
emission process is rather unlikely since Rh(III) is redox inert sys-
tem [60]. Although the nature of the emission level of these rho-
dium(III)–PAN compounds is not obvious, but can be attributed to
an ILCT level involving the donor and acceptor moieties of the
PAN ligand [55,56,61].
PAN is supported by the theoretical study where HOMOs as well
as LUMOs are almost exclusively distributed over the PAN ligand
for both compounds (vide infra).
4. Conclusion
In this work, mononuclear Rh(III) complex of PAN of the type
trans-[Rh(PAN)Cl(PPh3)2]PF6 (PAN = NNO donor azo ligand) is re-
ported. We were also able to isolate the intermediate cis-
[Rh(PAN)Cl2(PPh3)] with 1 equivalent of PPh3 relative to both 1
and RhCl3ꢀ3H2O. Both the complexes were characterized by various
spectral techniques and their structures were authenticated by X-
ray crystallography. The title ligand behaves as a tridentate mono-
anionic N,N,O-donor and shows strong affinity toward Rh(III), indi-
cating its overall borderline nature. The complexes thus formed
show rich spectral as well as redox properties, which have been
thoroughly investigated. Most notable is the appearance of absorp-
tion and luminescence in the low-energy part of the visible region.
Remarkably, the rhodium(III)–PAN complexes show red lumines-
cence in solution at room temperature. The redox orbitals of both
compounds are supposed to be centered on coordinated PAN
ligand.
3.4. Ligand redox
Electron-transfer property of the rhodium(III)–PAN complexes
has been studied by using cyclic voltammetry in acetonitrile
(ꢃ0.1 M NEt4ClO4) by using a platinum working electrode and
the reported potentials are referenced to the Ag/AgCl electrode.
Voltammetric data are presented in Table 3 and the voltammo-
grams are shown in Fig. 6. The value for the ferrocenium/ferrocene
couple, under our experimental conditions, was 0.52 V. Both com-
plexes show an oxidative response and the voltammograms are
irreversible in nature. The one-electron nature of the oxidation
processes has been verified by comparing theirs current height
(ipa) with that of the standard ferrocenium/ferrocene couple under
identical experimental conditions. It is very likely that this process
corresponds to the oxidation of the three-coordinated 1-(20-
pyridylazo)-2-naphtholate ligand (Eq. (2) and (3)). The oxidative
response can be assigned primarily
Acknowledgements
We thank the Department of Science and Technology, New Del-
hi, for financial assistance (Grant SR/FTP/CS-20/2005). We also
thank the Department of Chemistry, IIT Guwahati and DST-funded
National Single Crystal X-ray diffraction Facility at the Department
of Inorganic chemistry, IACS for X-ray data collections. Financial
assistance received from the UGC-CAS program in the Department
of Chemistry, Jadavpur University is acknowledged. We thank Dr.
K.K. Rajak, Department of Chemistry, Jadavpur University for help
in computations. B. Adhikari thanks the UGC, New Delhi, for his
fellowship.
½RhIIIðPANꢂIÞClðPPh3Þ ꢅþ ꢂ eꢂ ! ½RhIIIðPANÞClðPPh3Þ ꢅ2þ : for 3þ ð2Þ
2
2
½RhIIIðPANꢂIÞCl2ðPPh3Þꢅ ꢂ eꢂ ! ½RhIIIðPANÞCl2ðPPh3Þꢅþ : for 4
ð3Þ
to a ligand-centered redox process [26]. In addition, both complexes
show one-electron irreversible reduction waves (Epc) near ꢂ0.5 V
versus Ag/AgCl. These responses are believed to be centered on
the coordinated azo ligand (Fig. 5) [1,2]. The cyclic voltammogram
of neutral HPAN exhibits irreversible reductive response at
Appendix A. Supplementary data
CCDC 703322 and 703321 contain the supplementary crystallo-
graphic data for 3PF6ꢀCH2Cl2 and 4ꢀCH2Cl2. These data can be ob-
tained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk. 1H NMR spectra (Fig. S1)
and composition of frontier molecular orbitals (Table S1 and S2)
of 3PF6 and 4 are given. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.poly.2009.12.001.
ꢂ1.33 V versus Ag/AgCl corresponds to azo reduction of the free li-
*
gand. Analogous reduction of azo
p
MO of other free ligands incor-
porating ‘‘pyridylazo” moiety were observed near ꢂ1.30 V [62,63].
In complexes the azo reduction occurs at more positive potentials
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